Efforts in optimizing reducing agents, cysteine-HCl center dot H2O and sodium sulfide in order to attain satisfactory responses during acetic acid fermentation have been carried out in this study. Cysteine-HCl center dot H2O each with five concentrations (0.00-0.50 g/L) was optimized one at a time and followed by sodium sulfide component (0.00-0.50 g/L). Response surface methodology (RSM) was used to determine the optimum concentrations of cysteine-HCl center dot H2O and sodium sulfide. The statistical analysis showed that the amount of cells produced and efficiency in CO conversion were not affected by sodium sulfide concentration. However, sodium sulfide is required as it does influence the acetic acid production. The optimum reducing agents for acetic acid fermentation was at 0.30 g/L cysteine-HCl center dot H2O and sodium sulfide respectively and when operated for 60 h cultivation time resulted in 1.28 g/L acetic acid production and 100% CO conversion. (c) 2007 Elsevier Ltd. All rights reserved.